would I run into issues?
If you have the necessary power and you can connect two cards to your system, yes.
Usually mixing AMD/Nvidia GPUs on one system is easier on Windows. It can be done on most Linux distributions, but installing drivers is generally much more of a pain.
yeah was going to use windows.
now if I go this route can I use them both to mine 1 coin and or split the coins the cards to mine 2 coins each?
will I run into problems where one card cant mine one coin?
It is possible to do either, however the 1070 and Vega 56 will shine on different algorithms. For example, it may be better to mine an equihash coin with the 1070 and a cryptonight coin with the Vega, rather than to use both for the same coin.